Her husband and his brothers, David, Samuel and Andrew had settled a section in the Hoffman Bend area of what would become St. Clair County, Missouri, and had already been in some trouble with the authorities.

[1] After his brothers left for the west, David remained behind as David Kelso in Missouri, taking care of their elderly father, Samuel Kelsey Sr.[1] Nancy Kelsey was 17 years old in 1841 when Ben decided to travel west after reading doctor John Marsh's letter extolling the California climate and crop-growing advantages.

[9] Shortly thereafter on May 18, 1841, Ben and Nancy with their one-year-old daughter,[10] Martha Ann, and Kelsey's brothers joined John Bidwell as members of the first wagon train to California.

Father Pierre-Jean De Smet performed the ceremony for Nancy Kelsey's sister "Betsey Grey" and her new husband Richard Phelan before the two emigrant groups and the priest's parties split up in August 1841.

[16] Many years later, an interview with Nancy Kelsey was published in the San Francisco Examiner where she recollected the trip across the continent; she said the group had no guide and no compass.

[16] In 1843, while she was pregnant, they left Sutter's employ and drove cattle north along the California–Oregon Trail east of Mount Shasta, past Klamath Lake, and on to Oregon City.

[9] After selling their cattle, the family loaded up with goods and returned to California where they settled on 2,000 acres (810 ha) at Calistoga, neighboring General Vallejo's property to the south.

[16] Ben's brothers Isaac and Samuel with their families arrived in northern California at the end of 1843;[16] they had started with the Bidwell party but branched off for Oregon.

[23] While tensions were growing in the spring of 1846, Nancy delivered son Andrew Kelsey on April 7, bringing the total of living children to three.

[9] During the Bear Flag Rebellion in 1846, while the Kelsey brothers joined John C. Fremont in declaring California's independence from Mexico, Nancy Kelsey, Mrs. John Sears, and Mrs. Benjamin Dewell sewed the original "Bear Flag" from a pattern drawn by William L. Todd, a nephew of Mary Todd Lincoln.

[9] Ben left Nancy and the four children behind when he took fifty Pomo men from his brother Andrew's ranch southwest of present-day Kelseyville, on Rancho Lupyomi at the American River, in the Sierra foothills to mine for gold.

[20] Ben had killed a Native American chief on their overland trip to Humboldt Bay[22] and his reputation got to town before they did, a Eureka correspondent wrote: "Kelsey of Sonoma (the Indian killer) is on his way here with his own and several other families.

[20] The Kelsey National Recreation trail follows 8 miles (13 km) of the route between Scott River Road and Paradise Lake.

[9] Seeking drier weather for her husband, the family moved to Mexico in 1859, and then to Texas in 1861 where Nancy, Martha Ann and two younger daughters survived an attack by Comanche.

Her two oldest girls hid, but 12-year-old Mary Ellen was captured and found the next day scalped,[10]: 19–20  stabbed and with a fractured skull[21] which she survived but suffered mental illness until her death six years later[30] In 1864, the family returned to California,[10]: 20  at first in San Luis Obispo, then settling near Fresno in 1865.

[21] The wandering continued; the family moved to Inyo County, survived the 1872 Lone Pine earthquake, which Ben said was the only thing that ever frightened him.

[31] Nancy Kelsey outlived her husband and spent her final years on farms in Santa Barbara County, California, where she was a midwife and an herbalist.

In 1937 the Native Daughters of the Golden West put a bronze plaque on Nancy Kelsey's grave at the head of Cottonwood Canyon in the Cuyama River Valley.